Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin clay in paper coatings serves, for example, to improve the opacity of the coated paper. Opacity in such paper coatings is enhanced by the light scattering ability of the kaolin clay, and such ability can be estimated by the scattering coefficient as described in TAPPI, 1978, Vol. 61, No. 6, pages 78-80.
Calcined kaolin is a particular type of kaolin clay and is often used in large quantities for paper manufacture. Calcined kaolin can be obtained by heating beneficiated kaolin clay at temperatures of at least 600.degree. C. Calcined kaolin has improved light scattering characteristics (as compared to the non-calcined kaolin) and, therefore, contributes a higher degree of opacity to the paper coating. If kaolin clay is dehydrated by calcination, the calcined kaolin clay will not return to its naturally occurring hydrated form upon contact with water, but will retain its new characteristics.
As large quantities of calcined kaolin clay are often used in the paper and other industries, the transportation of such material to the customer is the subject of considerable effort. At present, calcined kaolin clay is commonly shipped as either a dry powder or an aqueous slurry having a solids content of 48-52%, by weight.
High solids slurries of calcined kaolin clay are desirable because tank cars and trucks could then be used for bulk shipments to increase cost effectiveness. However, calcined kaolin clay is dilatant in slurry form and, therefore, only moderate solids levels have previously been achieved on a commercial scale (i.e., 48-52% solids).
At moderate solids levels, two problems are frequently encountered with calcined kaolin clay slurries. First, settling occurs when these slurries are permitted to stand without agitation. Second, the rheology of these slurries can result in slurries which are too dilatant (i.e., too viscous) to be pumped by conventional means from tank cars or trucks.
High solids slurries of calcined kaolin clay are also desirable in paper manufacturing because increased coating solids, improved dryer efficiencies, improved coating properties and other advantages can be achieved.
Several methods have been proposed to provide high solids calcined kaolin slurries. For example, U.S. Pat. No. 3,754,712 subjects a suspension of calcined kaolin to wet-milling during which increments of calcined kaolin are added to the suspension until the desired high solids content is produced.
U.S. Pat. No. 4,017,324 provides an aqueous slurry of calcined clay and hydrated clay in which the total clay content is about 50-75%, by weight.
U.S. Pat. No. 4,118,245 provides a method for preparing stable high solids slurries of calcined kaolin clay in which a non-stick grinding medium is used.
U.S. Pat. No. 4,118,246 provides a stable high solids slurry of calcined clay through the use of one or more micromixers.
U.S. Pat. No. 4,374,203 provides a stable calcined clay slurry in which anionic and cationic polymers are present in the slurry.
However, for various reasons, the prior art fails to provide an acceptable process for producing high solids calcined kaolin slurries in which the rheological properties of the calcined clay slurries are improved and the opacifying properties are retained. Therefore, a need exists in the kaolin industry for this type of process.